Shared-tenant services arrangements are well known in the art. They permit a plurality of customer (i.e. tenant) groups, each one of which might otherwise require the services of its own, dedicated, private switching system, (e.g., different businesses within a building) to share the use of a single switching system (e.g., the building's private-branch exchange, or PBX). The shared-tenant services arrangements try to approximate dedicated switching systems in their interactions with individual customer groups and the services that they provide thereto. However, known commercially-available shared-tenant services arrangements have a serious disadvantage in that they require all customer groups to accept the overall limitations inherent in the switching system as if it were serving only a single customer group. Particularly, they require the extension numbering plans for the different customer groups (which together constitute a switching system's internal network numbering plan) to be mutually compatible and non-overlapping with each other.
U.S. Pat. No. 4,259,549 discloses an arrangement that attempts to remove this disadvantage and provide each customer group served by a single switching system with the capability of defining its own extension numbering plan that is independent of the extension numbering plans of the other customer groups. The arrangement has a set of extension translation tables for each customer group. Which set of translation tables is used to determine the intended meaning of a dialed extension is determined by the identity of the switching system physical port at which the extension is received.
While solving the problem of interdependence of the individual extension numbering plans, the proposed arrangement has a serious disadvantage of its own: it does not allow an individual group's members (i.e., switching system physical ports) access to any other extension numbering plan but their own. Consequently, members of one group cannot call members of another group in the same manner (e.g., by dialing the same extension numbers) as the members of the other group would call each other.
The above-referenced patent further illustrates a manner in which blocks of adjacent directory numbers may be made to call for the execution of identical system functions, whether such block of adjacent directory numbers are dialed from the same or different customer groups of stations. A limited degree of cross-plan access is achieved thereby between the different customer groups. However, this capability is achieved by means of overlap of the different customer groups' sets of translation tables, and hence only at the sacrifice of the desired independence of the numbering plans of the different customer groups. In other words, the patent fails to achieve simultaneously both numbering plan independence and cross-plan access from different customer groups.